drm/i915: add noop power well handlers instead of NULL checking them

Reading code free of special cases wins over the small overhead of
calling a noop handler. Suggested by Jesse.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/i915/intel_pm.c
Simple merge